Aerosol Filtration in Nanofiber Filter Media

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nano fibersNanofiber filter media are used in air filter applications where a compact filtration system with very high overall and fractional efficiency and low initial pressure drop  is required.  Moreover, because the aerodynamic drag force on nanofibers is very low compared to the drag on the large dust particles collected on the nanofiber media, this media is ideal for application to self-cleaning filtration.  The particles can be easily detached by applying reverse airflow or just shaking the filter.

Fiber diameter is the main variable responsible for filter efficiency and pressure drop.  Efficiency increases rapidly with decreasing fiber diameter.  The ratio of nanofiber diameter of commercial filter media to fiber diameter of the commonly used cellulose fibers is approximately equal to 1:150.  This results in an enormous surface area increase for nanofiber filter media.  Surface area for 100 nm nanofibers is approx. 40 square meters/gram of the fibers while only 0.2 square meters/gram for 20-micron cellulose fibers.

Knudsen number, knudsen number  (whereknudsen number= 65.3 nanometers = the mean free path of air molecules under standard conditions and is inversely proportional to the pressure; df= fiber diameter) is used to classify air flow regimes that are listed in the Table.

Free molecule

Transition

Slip flow
(Cunningham)

Classical

Kn >10

Kn = 10-0.25

Kn < 0.25

0.013 – 0.0013

df < 65 nm

df = 65-400 nm

df >400 nm

1-100 microns

Pressure drop, that is one of the major filter performance specification,  significantly increases with decreasing fiber diameter as a function of 1/df2, until the free molecule regime is reached where pressure drop is a function of 1/df.  However, this is a valid dependence only for clean filters.  When dust deposits form on nanofibers, this benefit of low-pressure drop diminishes with increasing amount of deposited dust.  Moreover, nanofibers capture very fine particles.  The pressure drop increases more rapidly for this compacted dust cake.  Therefore, it is important to design nanofiber filter media with significantly higher permeability than classical cellulose media to obtain long life filters.

Usually, the permeability of nanofiber filter media is 2-4 times greater than the value for the classical cellulose media, still providing great total and very high initial filter efficiency.  Combined mathematical models for granular and fibrous media should be used to optimize nanofiber filter media for specific dust cake filtration.

The picture shows: a) – initial filtration – dust particles are mainly captured by the uniformly distributed nanofibers with an average diameter of 150 nanometers.  Picture b) represents dust cake formation on nanofiber filter media, while picture c) shows excessive amount of nanofibers applied to the cellulose substrate.  Pressure drop of such media will rapidly increase like in the case of membrane filters.

Nanofibers

In air filtration with high aerosol velocity (greater than approximately 5-10 cm/s), the basis weight of 100-300 nanometer fibers should be less than 0.1 grams/ square meter, usually in the range of 0.3 – 0.7 grams per square meter. Cellulose or synthetic media can be used as a substrate.

 

Article by:

Tadeusz (Tad) Jaroszczyk
25 Cocopah Circle
Jim Thorpe, PA 18229-3713
tdjarosh@ptd.net
Tel. 570-732-4044

Dr. Jaroszczyk entered the field of motor vehicle air and liquid filtration in 1970.  Throughout his 39 years in the field, he held positions of Cummins Filtration Research Fellow, Vice-President of Motor Vehicle Institute, Manager of Aerosol Filtration Research, and other scientific and engineering positions.  He was responsible for development of Air Filtration Research Laboratory, Engine Air Intake Filters, Cabin Filters, Filters for Gas Turbine Intake Systems, and filters for industrial machine closed ventilation/filtration systems.  Recently, Tad has developed the idea of the family of Direct Flow filters – he is a leading author of six US patents on this subject.  He also managed the development of a depth-type, multi-layered synthetic filter media for engine coalescing filtration and upgrading the technology for producing this media.  Tad started to focus on science and engineering of nanotechnology in 2000.

Tad led the development of three advanced air filtration laboratories in Europe and two world class air filtration laboratories in the USA.  He published more than 100 technical papers (56 in English and 57 in other languages), including one book and one book chapter on separation/filtration science and technology he regularly teaches short internal and external courses on the subject.  He holds more than 25 patents including 15 US patents.  Tad organized and chaired SAE J1669 Passenger Compartment Air Filter Test Code Subcommittee.  He also organized and led SAE Engine Fractional Efficiency Test Code Subcommittee.  He was awarded a Certificate of Appreciation in recognition of outstanding service from the SAE in 1998.  Tad received Presidential Technology Award from Cummins Filtration President in 2006.  The same year, he received Frank Tiller and Fellow Member Awards from the American Filtration and Separations Society (AFS).  He received AFS Lifetime Achievement Award in 2008.  Tad co-organized three AFS international conferences, and chaired many filtration sessions at SAE, AFS, Filtech, and European KONES.  He is a Fellow Member of AFS and European Kones and a member of several professional societies.  He completed his 17-year military service with the rank of Lt. Col.  He obtained P.E. license State of Wisconsin in 1994.  Tad retired from Cummins Filtration Inc, Stoughton, Wisconsin on December 31, 2008.